Recently, various image forming apparatuses such as an MFP (Multi Function Peripheral) using an electrophotographic process system have come into widespread use, and typically, a laser printer for reproducing tone by driving laser on the basis of a digitized image data and the like are given. Furthermore, image forming apparatuses such as a color printer for reproducing a color image by repeating the process mentioned above for each of three or four colors or by performing tandem process have spread rapidly.
In such image forming apparatuses, a record area of toner is decreased by shaping a contour part of an image to be thin, and thus it is possible to reduce consumption of ink. Therefore, for the purpose of saving the toner, there has been developed a line thinning technique of, for example, performing pattern replacement or determining a contour pixel as a sample point and forming the contour on the inner side by interpolation.
However, such a technique is mainly based on a character type document. Hence, when this technique is applied to an image in which characters and photos both exist, the problem such as deterioration of the tone reproduction arises. The reason is that a pseudo contour or the like occurs when the photo area is n-valued by a toning method such as a dithering and the contour of halftone dots representing a photo is cut off.
Accordingly, there has been developed a technique of performing a line thinning only on thick lines or solid parts by separating character areas and photo areas, thin lines and the thick lines, or isolated points and the solid parts.
On the other hand, recently, resolution of these image forming apparatuses has increased, and image forming apparatuses having a high resolution, for example, 1200 dpi have been embodied. Alternatively, in order to increase image quality and resolution, a multi-level PWM technique has been employed increasingly to reproduce images by performing a multi-level toning on one pixel.
Furthermore, by setting a high resolution in a RIP processing for converting font and graphic data into raster data having a relationship with positions of an actual output image, it is possible to achieve high image quality and high precision. For example, a contour part forming an image of 1200 dpi is smoother than that of 600 dpi. Image reproduction quality of detailed parts of an image is improved also on the basis of a sampling theorem. In addition, since the generated fonts and graphic data is smooth, in such a line thinning, high resolution in an output device enables more precise 2-dimensional control, thereby acquiring higher quality output as a result.
However, the known line thinning is generally performed on a binary image, and thus it is hard to perform the line thinning on an image which is processed by a multi-level toning to increase precision and image quality. The reason is that the image processed by the line thinning does not correspond to the multi-level halftoned image.
Meanwhile, when resolution of an image forming apparatus increases, for example, when the identical speed is achieved with the identical number of beams, an image clock speed extremely increases, thereby increasing cost of the apparatus. Because of the reason, a problem arises that the image forming apparatus has a very high price. Regarding to the problem, the method of decreasing a frequency of the image clock by employing a laser array in an optical system can be considered. However, in this case, additional hardware costs for controlling the laser or an optical device increase.